The forum now has an option to resize images to 320x240 pixels using "rimg" tags.
This gives a large enough image to display clearly but doesn't cause a problem for those with slow connections
You can insert a pair of such tags with the "rimg" button, which sits rightmost on the row of buttons above the text field where you type your message.

Upload your image to an image hosting site then paste the URL of the "Direct" or "Download Image" link between the rimg tags.

Here is one from LM 17 Cinnamon. For anyone interested the wallpaper is from wallbase.fr, (walk the ) plank PPA is here https://launchpad.net/~ricotz/+archive/ubuntu/docky and I put a screen shot of the theme choices, plus I used Crunchy-grey for Cinnamon.